Lunacy, n.; pl. Lunacies.
Etymology: [See Lunatic.]
1. Insanity or madness; properly, the kind of insanity which is broken by intervals of reason, -- formerly supposed to be influenced by the changes of the moon; any form of unsoundness of mind, except idiocy; mental derangement or alienation. Brande. Burrill. Your kindred shuns your house As beaten hence by your strange lunacy. Shak.
2. A morbid suspension of good sense or judgment, as through fanaticism. Dr. H. More.
Syn. -- Derangement; craziness; mania. See Insanity.
Shades of meaning
Fernald has no entry of its own for lunacy; he lists it under insanity.
Where he writes about it:
Lunacy originally denoted intermittent insanity, supposed to be dependent on the changes of the moon (L. luna): the term is now applied in general and legal use to any form of mental unsoundness except idiocy. — from insanity
In the derived sense, lunacy denotes what is insanely foolish, madness what is insanely desperate. — from insanity